The present invention relates to spectrometry, and more particularly, to spectrometer devices providing chemical analysis by aspects of ion mobility in an electric field.
Chemical detection systems are used in a wide array of applications. These devices may take samples directly from the environment, or may incorporate a front end device to separate compounds in a sample before detection. There is particular interest in providing a chemical detection system capable of accurate compound detection and identification, and which may be deployed in various venues, whether in the lab, in the workplace or in the field.
Mass spectrometers are well known as the gold standard of laboratory-based chemical identification. But they are relatively difficult to deploy in the field. Such systems operate at low pressures, resulting in complex systems, and the spectra output can be difficult to interpret, often requiring a highly trained operator.
At times a gas chromatograph (GC) is used as a front-end to an MS, with good results. But the GC-MS is not well-suited for small, low cost, fieldable instruments for real-time chemical detection. Nevertheless there is a continuing need for fieldable instruments generation of real-time detection data.
Detection of species of NOx is a good example of this need. It is well known that reactive nitrogen oxide species NOx such as NO, NO2, and NO3 play a major role in atmospheric chemistry. These species are important in the ozone and nitrogen cycles, which produce detrimental photochemical smog and acid rain. Tougher environmental regulations to reduce these levels in the atmosphere require higher performance detectors and monitors applied to anthropogenic sources of NOx (e.g. exhaust from internal-combustion engines, steel mill processing, power plant emissions, etc.) with higher sensitivity and faster response times. Enhancement of fuel economy of internal-combustion engines is another driver for the development of sensors which are able to precisely, and rapidly, monitor NOx levels in exhaust emissions.
The medical value of detection of NO in exhaled breath has also been recognized for clinical diagnosis. For example, it has been reported that such clinical analysis can provide a noninvasive window into the activities of disease, such as asthma, chronic obstructive pulmonary disorder, and cystic fibrosis, in the lower airways. There is therefore a desire for improved, portable and simple apparatus and method for evaluation of NO in exhaled breath for medical purposes.
There are a number of reliable measurement techniques which have been developed for monitoring nitrogen oxide species: These include ion-based detection, chemiluminesence, electrochemical, acoustic gas sensors, and ZrO2 solid electrolyte sensors, laser systems, and the like. These techniques, however, generally require sophisticated optical equipment, or suffer from significant drawbacks such as slow response times, or the detection of only certain NOx species, making them problematic for routine measurements.
Ion Mobility Spectrometry (IMS) has been explored recently as an approach to realizing a more sensitive, selective and robust device for NOx monitoring. In dry (humidity˜10 ppm) operating conditions, the IMS shows high sensitivity (10's of ppms) and fast response times (10's of ms). However, a serious disadvantage is that its response is highly affected by the presence of moisture. For example, in one demonstration, a level of 3% humidity completely suppressed IMS response to a sample at 483 ppm NO2.
Time-of-flight Ion Mobility Spectrometers (TOF-IMS) are considered to be functional chemical detectors. High-speed response and low memory effects have been attained, and the gas phase ion chemistry inside the TOF-IMS can be highly reproducible. Widespread use, however, still remains a problem for TOF-IMS. Among other things, TOF-IMS flow channels (also referred to as drift tubes) are still comparatively large and expensive and suffer from losses in detection limits when made small.
The differential ion mobility spectrometer ((DMS), also known as a high field asymmetric waveform ion mobility spectrometer (FAIMS)), is an alternative to the IMS. In a DMS device, a gas sample that contains a chemical compound is subjected to an ionization source. Ions from the ionized gas sample are drawn into an ion filter region, where the ions flow in a compensated high asymmetric RF field generated between filter electrodes, the field being transverse to the ion flow. The field is compensated to allow selected ion species to pass through the filter, based on aspects of their mobility in the field. These ion species are passed downstream to an ion detector. Detections are correlated with field conditions and compensation and species identification is made by reference to known species behavior in the extant compensated DMS field.
The asymmetric field alternates between a high and low field strength condition that causes the ions to move in response to the field according to their mobility characteristics. Typically the mobility in the high field differs from that of the low field. That mobility difference produces a net transverse displacement of the ions as they travel in the gas flow through the filter. This transverse travel of the ions continues until they drive into one of the filter electrodes and are neutralized. However, the field is also compensated such that a particular ion species will remain toward the middle of the flow in the flow path and will pass through the filter without neutralization. The amount of change in mobility in response to changes in the asymmetric field is compound-dependent. This permits separation of ions from each other according to their species based on the applied compensation (usually a dc bias applied to the filter electrodes).
In the past, Mine Safety Appliances Co. (MSA) made an attempt at a functional cylindrical FAIMS device with coaxial electrodes, such as disclosed in U.S. Pat. No. 5,420,424. (This FAIMS technology is referred to by MSA as Field Ion Spectrometry (FIS).) The device has been found to be complex, with many parts, and somewhat limited in utility.
It is a therefore an object of the present invention to provide a functional, small, spectrometer that overcomes the limitations of the prior art.
It is another object of the present invention to provide a chemical sensor with fast response times for real-time process control, especially for detection and identification of NOx related species in real-time.
It is another object of the present invention to provide a chemical sensor for detection and identification of NOx related species in real-time with minimized effect of moisture upon detection results.
It is yet another object of the present invention to provide low cost and compact, reliable instrumentation that is useful for laboratory and field conditions and is capable of making in situ measurements of chemicals present in complex mixtures at various venues.